A wide variety of diseases associated with the human and animal ears have been identified. The more frequently diagnosed pathologies include obstruction of the external ear canal, atresia of the external ear canal, perforation of the tympanic membrane, retraction of the tympanic membrane, otitis in its various forms (adhesive, purulent and non-purulent), otosclerosis, fixation of the stapes, and cholesteatoma, among others. In children, otitis media is one of the most common pathologies. By itself, otitis media is a significant affliction which can lead to serious long-term hearing and learning disabilities if not promptly diagnosed and treated. Further, otitis media is frequently symptomatic of other pathologies, and thus useful in their diagnosis.
These ear pathologies are generally diagnosed using common diagnostic techniques, such as tympanometry, pneumatic otoscopy or visual otoscopy. While the usefulness of these techniques is well-recognized and established, these techniques do have some difficulties. For example, with both tympanometry and otoscopy, personnel who conduct tests and interpret results must be highly trained. Since these techniques cannot be performed by non-medical or inexperienced personnel, efficient screening of children or infants at home or in a school is not possible with these techniques.
Additionally, a patient must cooperate with the personnel performing these techniques, but patients subjected to these techniques may experience considerable discomfort. In particular, discomfort from tympanometry or pneumatic otoscopy arises since (1) an airtight seal is required to obtain useful measurements, (2) any probe assembly must be inserted deep into the ear canal, and (3) the air pressure in the ear canal must be varied above and below atmospheric pressure to obtain useful measurements.
The diagnosis of otitis media in young children using common diagnostic techniques is particularly difficult because of the fear, discomfort or even pain, associated with many of these techniques. The usefulness of examination by conventional techniques is often diminished because discomfort of the child typically leads, at best, to movement by the child which impairs the examination and, at worst, to a refusal to allow the examination to proceed. The problem is especially acute when the examination is made in a mass screening environment, such as may take place in hospital clinics where large numbers of patients must be seen in a comparatively short time.
Many of the problems with these common diagnostic techniques were overcome by a device which measures acoustic reflectance, which is a quantity related to the complex acoustic impedance of the middle ear. A suitable device and methodology for measuring acoustic reflectance are disclosed and described in U.S. Pat. Nos. 4,601,295 and 4,459,966 to John H. Teele (the Teele patents). Such devices were made commercially available by ENT Medical Devices, Inc., of Wareham, Mass., and Endeco, Inc., of Marion, Mass. In the literature, this diagnostic technique is generally referred to as acoustic reflectometry and the device is generally referred to as an acoustic reflectometer or acoustic otoscope.
Acoustic reflectometry involves transmitting sound waves (called incident waves) through the ear canal to the tympanic membrane. Some of the incident waves are reflected off the tympanic membrane and other components of the ear. The incident waves are selected from a range of frequencies including the resonance frequency of the tympanic membrane; ideally the amplitudes of the incident waves are also the same but this is often not achievable. The vector sum of these reflected waves with the incident waves is obtained by a microphone. The envelope of the vector sum of the incident and reflected waves over the range of frequencies, herein called an acoustic reflectance curve, has a dip, also called a null. The peak of this dip, actually a minimum, is known as a null value. In the literature and in its commercial use, acoustic reflectometers calculate an acoustic reflectance curve for an ear and detect the presence and frequency centerline of the dip and the null value. The null value is the primary basis for diagnosis of ear pathologies. Although the Teele patents state that "shape" of the characteristic dip can be detected along with the presence, frequency and amplitude of the dip, these patents do not discuss significance of the shape of the characteristic dip other than that shape means how pronounced or sharp the dip is. The Teele patents do not discuss how shape of the dip is detected or measured or how it is used in diagnosis.
One of the commercially available acoustic reflectometers was a "T"-shaped device which provided the amplitude of the null value and the incident frequency at which it occurred using a set of horizontal diodes to indicate the frequency at which a dip occurred, and a set of vertical diodes to indicate the null value. This device, the Model 501 Acoustic Otoscope from ENT Medical Devices, Inc., could be equipped with a recorder, or printer, that allowed a visual representation of the entire acoustic reflectance curve to be viewed.
Several articles in the literature describe how the null value of an acoustic reflectance curve obtained using commercially available devices is correlated with ear pathologies. In particular, severe middle ear effusion (MEE) generally causes high null values, whereas normal ears cause low null values. However, there is a significant range of measurements for which the diagnosis is uncertain, e.g., probable MEE such as when an effusion is just beginning to develop. Such a range of uncertainty limits the sensitivity and specificity of the process and device. The literature also includes several studies which reach a variety of conclusions on the specificity and sensitivity of the device for diagnosing MEE.
It was discovered that the accuracy of the measurement of the null value obtained with an acoustic reflectometer depended on the line of sight from the instrument tip to the tympanic membrane. A direct line of sight provides the most accurate results. When a direct line of sight is not obtained, due to improper aiming or because of the ear itself, measurements of the null value are less likely to indicate an unhealthy ear and are more likely to fall in the range of uncertainty, indicating only probable MEE. An unhealthy ear may be diagnosed as healthy.
Due to this range of uncertainty, commercially available acoustic otoscopes were eventually provided with operating instructions that directed a user to look at the overall shape of the dip, when the acoustic otoscope was used with a recorder or printer. It was stated that, given detection of a null value in the uncertain range, a somewhat rounded dip suggests a dry ear condition or negative pressure behind the tympanic membrane but no effusion. It was further states that a sharply peaked dip suggests a condition where the middle ear is partly air-filled, partly fluid-filled.
These operating instructions were based primarily on a study by Jerome T. Combs, entitled, "Predictive value of the angle of acoustic reflectometry," in The Pediatric Infectious Disease Journal, vol. 10, no. 3, pp. 214-216, March 1991. This article states that an "angle" formed at the null of the acoustic reflectance curve, as displayed on the Model 501 Acoustic Otoscope with recorder, is useful in combination with the null value to distinguish healthy ears from unhealthy ears where the null value is inconclusive. Angle measurements were performed manually on the printout using a protractor or goniometer. The paper does not describe any controlled procedure by which points or lines on the printed acoustic reflectance curve were selected to define the angles being manually measured.
The article describes a study in which acoustic reflectance measurements were obtained for 406 ears of 203 children (96 girls and 107 boys) between the ages of 4 and 16 using the Model 501 Acoustic Otoscope with recorder. Of this number, there were 75 ears of tympanometry, 149 ears of tympanometry and 182 ears of tympanometry. The purpose of the study was to determine whether the "angle" formed by the dip in the acoustic reflectance curve had any predictive value. Although the article does conclude that "angle" apparently has some predictive value, there are two problems with the study that suggest that the results lack adequate statistical significance to be conclusive about this predictive value. First, the number of subjects analyzed was arbitrarily selected. This actual number of subjects is statistically insignificant. A much greater sample would be more statistically persuasive. Second, the angle formed by the dip was measured manually using a protractor on a printout of the acoustic reflectance curve. Since the paper lacks a description of a deterministic method for establishing the points defining the angle, the angle measurements are likely to have a fair amount of a variance in them, further weakening the statistical significance of the results.
While an acoustic reflectometer is a useful diagnostic tool, there remain some unsolved problems in achieving accurate diagnoses. First, inexperienced personnel are more likely to obtain inaccurate results because accurate measurement of the null value of acoustic reflectance still requires a direct line of sight from the tip of the acoustic reflectometer to the tympanic membrane. Second, the ears of young children reflect less of the incident waves than those of older children, given the same ear pathology. In particular, children under six months of age have a tympanic membrane which is at a relatively shallow angle to the ear canal. In some cases, this position of the tympanic membrane prevents a direct line of sight from being obtained. These two factors may result in a measurement of the null value in a "healthy" range for an unhealthy ear.